1. Field of the Invention
The present invention relates to a read only memory (hereinafter referred to as ROM) device and the method for making the same in which the presence/absence of the electrical contact between two electrodes arranged overlapping with each other constitutes the stored data. More specifically, it relates to a ROM device and the method for making the same in which the writing of the stored data is simple and the reading of the stored data can be carried out at high speed.
2. Description of the Prior Art
An example of a conventional typical ROM device is disclosed in, for example, Digest of Technical Papers of IEEE International Solid State Circuits Conference (1972, pp. 54-55 and pp. 152-153).
In such a conventional ROM device, the data is stored dependent on the presence/absence of the connection at the intersection between a gate electrode of a MOS transistor and the electrode wiring. FIGS. 1 and 2 are vertical sectional views of the intersecting portion. FIG. 1 shows a connected intersection and FIG. 2 shows a non-connected intersection. A reference numeral 51 denotes a silicon substrate. 52 denotes silicon dioxide layers (insulating layers) interposed between the silicon substrate 51 and each of the electrode wiring 53 and 54 and between each of the electrode wirings 53 and 54 and a signal line 55. 56 denotes a hole selectively formed on the insulating layer 52 for conducting the intersection.
In this conventional ROM device, the presence/absence of the connection between the gate electrode of the MOS transistor and the electrode wiring, that is, the presence/absence of the hole 56 is read as a stored data. Therefore, in order to store the data, a hole 56 should be selectively formed beforehand. Therefore, prior to the step of arranging electrodes of each signal line 55, the insulating layer is processed to form the hole 56 employing a number of masks corresponding to the data pattern.
As described above, since the conventional ROM device stores data by the presence/absence of a hole 56 in the insulating layer 52, it took long time for processing the insulating layer 52, causing higher cost. In addition, the conductivity of the electrode constituting signal line 55 is not sufficient for required high speed reading.